1. Field of the Invention
The present invention relates to a probe device for measuring electric characteristics of objects to be tested such as ICs or LSIs formed on a semiconductor wafer, and, in particular, to a probe device having probes formed by a micro-strip line technique for measuring high-frequency electric characteristics of the objects.
2. Description of the Prior Art
A number of electrodes are arranged on a semiconductor device such as an IC or LSI, or on a liquid crystal display substrate (LCD substrate). In order to test the electric characteristics of such electrodes, a probe device is employed. In this probe device, a number of probe needles are disposed on a probe card with a predetermined arrangement. The test of an object is carried out in a state wherein the tip end portions of the probe needles are brought into contact with the electrode pads of the object.
A higher degree of integration in recent semiconductor devices such as ICs or LSIs has led to an increase in the number of electrodes in semiconductor devices and to a decrease in size of electrodes. In accordance with this tendency, the probe needles of a probe device need to be reduced in size and increased in density. The reduction in size and increase in density of probe needles has inevitably made the manufacture and mounting of probe needles troublesome. In particular, in the case where a coaxial cable structure is employed in a probe needle in order to enhance the high-frequency characteristic of a probe device, the manufacture and mounting of small, high density probe needles has become very difficult. This difficulty results in a low reliability of the probe device.
It is also required that an automatic test be performed with the use of a probe device. When high-integration semiconductor devices such as ICs or LSIs, having a number of small and densely arranged electrodes, are to be tested, a probe device having a probe card with conventional probe needles has various disadvantages, because of the characteristics of the probe needles. For example, in this type of conventional probe device, exact positioning of the needles must be performed prior to a test.
As a technique for overcoming the drawbacks of the conventional probe device, Japanese Patent Publication No. 61-14659 discloses a structure wherein probe needles are produced by a micro-strip line technique. Basically, a micro-strip line has a structure of "metallic strip-insulation base plate-metallic strip", and the micro-strip line technique is well known as a wiring technique for transmission of high-frequency signals.
In the technique disclosed in Japanese Patent Publication No. 61-14659, micro-strip lines, the number of which is identical to the number of electrode pads on an object to be tested, are individually attached to the bottom surface of a test base plate or a probe card. A distal end portion of each micro-strip line is arranged so as to project into an opening formed at the center of the probe card. The distal end portion of each microstrip line serves as a probe, and the distal end portion of the lead strip of each line functions as a contact for each probe. A jig for bringing the distal end portions of the micro-strip lines or probes into pressurized contact with the electrodes on the object is provided above the opening. A common shock absorber is interposed between the jig and the micro-strip lines. This structure is advantageous in that the high frequency measuring characteristic can be enhanced by the use of a micro-strip line structure and the troublesome step of, for example, fine positioning of probes, as in the prior art, can be made unnecessary.
Owing to the high integration of semiconductor devices such as ICs or LSIs, the length of one side of a pad serving as an electrode in a semiconductor device between adjacent pads is about 70 to 110 .mu.m. In accordance with the size of the pad, the width of a microstrip probe is set to about 50 to 80 .mu.m. Under this circumstance, the desirable electric contact cannot be achieved by the above structure, wherein a common shock absorber is interposed between all micro-strip probes and the jig, thereby pressing each micro-strip line, which has a width of the unit of .mu.m, onto each electrode pad. This is because the electrode pads have uneven heights and the levels of adjacent pads are not equal.
In addition, in order to achieve a stable characteristic impedance in a micro-strip line, it is desirable that the width of the ground strip, located opposite each lead strip, which is put in contact with the electrode pad, be as large as possible. In an actual probe device, it is necessary to make the width of the ground strip sufficiently larger than the width of the lead strip. However, in the above conventional structure, the width of the lead strip is substantially equal to that of the ground strip.